1. Field of the Invention
The present invention is in the technical field of papermaking, and more particularly in the technical field of wet-end additives to papermaking furnish.
2. Description of the Prior Art
In the manufacture of paper an aqueous cellulosic suspension or slurry is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1 percent, and often below 0.5 percent ahead of the paper machine, while the finished sheet must have less than 6 weight percent water. Hence, the dewatering aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
The least costly dewatering method is drainage, and thereafter more expensive methods are used, for instance vacuum pressing, felt blanket blotting and pressing, evaporation and the like, and any combination of such methods. Since drainage is both the first dewatering method employed and the least expensive, improvement in the efficiency of drainage will decrease the amount of water required to be removed by other methods and improve the overall efficiency of dewatering and reduce the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of manufacture is retention of furnish components on and within the fiber mat being formed during papermaking. A papermaking furnish contains particles that range in size from about the 2 to 3 millimeter size of cellulosic fibers to fillers measuring only a few microns. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed.
One method of improving the retention of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the use of a coagulant/flocculant system, added ahead of the paper machine. In such a system there is first added to the furnish a coagulant, for instance a low molecular weight cationic synthetic polymer or a cationic starch, which coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly cellulosic fines and mineral fillers, and thereby agglomerates such particles. The coagulant is followed by the addition of a flocculant. The flocculant is generally a high molecular weight anionic synthetic polymer which bridges the particles and/or agglomerates, from one surface to another, binding the particles into large agglomerates. The presence of such large agglomerates in the furnish increases retention The agglomerates are filtered out of the water onto the fiber web, where unagglomerated particles otherwise would to a great extent pass.
While a flocculated agglomerate generally does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained an amount of gelatinous material, when such flocs are filtered by the fiber web the pores thereof are reduced, thus reducing drainage efficiency. Hence, the retention is increased at the expense of decreasing drainage.
Another system employed to provide an improved combination of retention and dewatering is described in U.S. Pat. Nos. 4,753,710 and 4,913,775, inventors Langley et al., issued respectively Jun. 28, 1988 and Apr. 3, 1990, the disclosures of which are incorporated hereinto by reference. In brief, such method adds to the aqueous cellulosic papermaking suspension first a high molecular weight linear cationic polymer before shearing the suspension, followed by the addition of bentonite after shearing. The shearing generally is provided by one or more of the cleaning, mixing and pumping stages of the papermaking process, and the shearing breaks down the large flocs formed by the high molecular weight polymer into microflocs, and further agglomeration then ensues with the addition of the bentonite clay particles.
Another system uses the combination of cationic starch followed by colloidal silica to increase the amount of material retained on the web by charge neutralization and adsorption of smaller agglomerates. This system is described in U.S. Pat. No. 4,388,150, inventors Sunden et all, issued Jun. 14, 1983.
Dewatering generally, and particularly dewatering by drainage, is improved when the pores of the paper web are less plugged, and it is believed that retention by adsorption in comparison to retention by filtration reduces pore plugging.
Greater retention of fines and fillers permits a reduction in the cellulosic fiber content of the paper being formed. As pulps of less quality are employed to reduce papermaking costs, the retention aspect of papermaking becomes more important because the fines content of such lower quality pulps is generally greater than that of pulps of higher quality.
Greater retention of fines, fillers and other slurry components reduces the amount of such substances lost to the white water and hence reduces the amount of material waste, the cost of waste disposal and the adverse environmental effects therefrom.
Another important characteristic of a given papermaking process is the formation of the paper sheet produced. Formation is determined by the variance in light transmission within a paper sheet, and a high variance is indicative of poor formation. As retention increases to a high level, for instance a retention level of 80 or 90 percent, the formation parameter generally abruptly declines from good formation to poor formation. It is believed that as the retention mechanisms of a given papermaking process shift from filtration to adsorption, the deleterious effect on formation, as high retention levels are achieved, will diminish, and a good combination of high retention with good formation is attributed to the use ofbentonite in U.S. Pat. No. 4,913,775.
It is generally desirable to reduce the amount of material employed in a papermaking process for a given purpose, without diminishing the result sought. Such add-on reductions may realize both a material cost savings and handling and processing benefits.
It is also desirable to use additives that can be delivered to the paper machine without undue problems. An additive that is difficult to dissolve, slurry or otherwise disperse in the aqueous medium may require expensive equipment to feed it to the paper machine. When difficulties in delivery to the paper machine are encountered, the additive is often maintained in aqueous slurry form by virtue of high energy input equipment. In contrast, additives that are easily dissolved or dispersed in water require less energy and expense and their uniformity of feed is more reliable.
The treatment of an aqueous cellulosic slurry with a cationic polymer followed by shear, preferably a high degree of shear, is a wet-end treatment in itself known in the field, for instance as described in U.S. Pat. Nos. 4,753,710 and 4,913,775, inventors Langley et al., issued respectively Jun. 28, 1988, and Apr. 3, 1990, the disclosures of which are incorporated herein by reference. The present invention departs from the disclosures of these patents in the use of a low molecular weight anionic polymer after the shear, instead of bentonite. Also, it has been found in this case that equivalent or greater activity can be found by reversing the order of addition of the compounds, i.e., introducing the anionic polymer prior to the cationic polymer.
As described in the Langley patents, paper or paper board is generally made from a suspension or slurry of cellulosic material in an aqueous medium, which slurry is subjected to one or more shear stages, which stages generally are a cleaning stage, a mixing stage and a pumping stage, and thereafter the suspension is drained to form a sheet, which sheet is then dried to the desired, and generally low, water concentration. As disclosed in these patents, the cationic polymer generally has a molecular weight of at least 500,000, and preferably the molecular weight is above 1,000,000 and may be above 5,000,000, for instance in the range of from 10 to 30 million or higher. The cationic polymer is substantially linear; it may be wholly linear or it can be slightly cross linked provided its structure is still substantially linear in comparison with the globular structure of cationic starch. Preferably the cationic polymer has a relatively high charge density of for instance about 0.2 and preferably at least about 0.35, and most preferably about 0.4 to 2.5 or higher, equivalents of cationic nitrogen per kilogram of polymer. When the polymer is formed by polymerization of cationic, ethylenically unsaturated monomer, optionally with other monomers, the amount of cationic monomer will normally be above 2 mole percent and usually above 5 mole percent, and preferably above 10 mole percent, based on the total moles of monomer used in forming the polymer. The amount of the cationic polymer employed in the process, in the absence of any substantial amount of cationic binder, is typically at least 0.005 percent based on dry weight of the slurry, and preferably 0.6 percent in the substantial absence of cationic binder and 0.5 percent in the presence of cationic binder, same basis, which is from 1.1 to 10 times, and usually 3 to 6 times, the amount of cationic polymer that would be used in conventional (dual polymer) processes, and hence is considered "an excess amount" of cationic polymer. The cationic polymer is preferably added to thin stock, preferably cellulosic slurry having a consistency of 2 percent or less, and at most 3 percent. The cationic polymer may be added to prediluted slurry, or may be added to a slurry together with the dilution water.
Also as described in aforesaid patents, the use of the excess amount of synthetic cationic polymeric flocculant is believed necessary to ensure that the subsequent shearing results in the formation of microflocs which contain or carry sufficient cationic polymer to render at least parts of their surfaces cationically charged, although it is not necessary to render the whole slurry cationic. Thus the Zeta potential of the slurry, after the addition of the cationic polymer and after the shear stage, may be cationic or anionic.
The present invention shows that low molecular weight cationic polymers may be used in conjunction with the anionic polymer of U.S. Pat. No. 5,098,520, the disclosure of which is incorporated herein by reference. In U.S. Pat. No. 5,098,520 the cationic polymer is limited to a molecular weight of 1,000,000 and higher.